Make it possible to compile 5.12 branch with a kdecoration from master

Summary:
KDecoration has two new pure virtual methods. Due to that one cannot
build KWin in the stable branch. This makes bug fixing rather cumbersome
as one also needs to switch the kdecoration branch.

This rather unusual change for a stable branch improves the
compatibility by adding the new virtual methods without doing anything
and without marking them as override, so that the code can compile with
a kdecoration from 5.12 and a kdecoration from master.

Test Plan: 5.12 branch compiles with kdecoration master branch

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10296